Such a beam tube is intended in particular for a particle accelerator for charged particles. The charged particle beam may for example comprise electrons, atomic nuclei, ionized atoms, charged molecules or charged molecular fragments. The acceleration of the charged particle beam takes place in a beam-guiding cavity, which is enclosed by the beam tube. The cavity is conventionally evacuated during operation of the particle accelerator. To this end, a vacuum pump system assigned to the beam tube is conventionally provided.
The beam tube, which separates the cavity and the charged particle beam from the surroundings, has the accelerating electric field electrostatically applied to it. With an increasing field strength of the electric field, the probability that stray electrons will be torn from the surface of the inner wall of the beam tube increases. This process takes place initially and preferentially on so-called whiskers. Whiskers are needle-shaped single crystals with a diameter of a few micrometers and a length of up to several hundred micrometers, which occur on all surfaces, in particular metallic surfaces. An increased electric field occurs at the tip of a whisker. Stray electrons are thereby torn from the tip of the whisker. The stray electrons are then accelerated by the electric field, just like the charged particle beam. If such stray electrons strike the inner wall of the beam tube, then secondary electrons will be detached by the impact. The process is self-sustaining. Finally, arcing takes place on the inner wall, and therefore collapse of the electric field accelerating the charged particles.
In order to resolve this problem, U.S. Pat. No. 6,331,194 B1 discloses a beam tube in which the cavity guiding the particle beam is directly enclosed by a hollow cylindrical isolation core, which is referred to as a high gradient insulator, HGI. The isolation core comprises a number of thin rings made of a dielectric (thickness about 0.25 mm), which are respectively provided with a thin metallically conductive layer (thickness about 40 000 angstroms) on their main faces. In order to produce the isolation core, the rings are assembled to form a hollow cylinder. Under the effect of pressure and heat, the adjacent metal layers of neighboring rings melt and fuse to form metal rings.
The HGI increases the sparkover resistance of the beam tube. This is because if secondary electrons are formed on the inner wall of the HGI, then the neighboring metal rings of the HGI become charged. The electric charge is therefore distributed over all metal rings directly exposed to the secondary electrons. This leads to homogenization of the electric charge on the inner wall of the HGI, and therefore a reduced tendency to secondary electron multiplication.
The distribution of the electric charge over neighboring thin metal rings is purely capacitive distribution. The principle therefore works only for infrequent and short voltage pulses. Charging of the metal rings is not effectively prevented, since the metal rings are embedded in the dielectric of the insulator core and the applied charge can only flow away slowly through leakage paths. Operation of the linear accelerator with a high rate of acceleration pulses therefore leads to an increasing sparkover probability.